There are approximately 40 biocontrol products commercially available for the control of plant diseases worldwide. Biocontrol products are available to control many diverse pathogens, as recently reviewed by Fravel, et al., “Availability and Application of Biocontrol Products,” Biological and Culture Tests for Control of Plant Diseases, 11:1-7 (1996). At least 27 genera of fungi, 3 genera of bacteria, and 4 genera of nematodes are targeted for control by these products. More than half of these products control soilborne fungi. The biocontrol agents themselves are also diverse and include at least 9 genera of fungi, 4 genera of bacteria, and one actinomycete. Biocontrol products are used on a great variety of crops including greenhouse crops, row crops, field crops, perennial field crops, and trees and wood, as well as in special cropping systems such as mushroom cultivation. The products are applied in many ways. They may be sprayed onto plants or harvested fruits, drenched on harvested fruit or on plants, incorporated into the soil, applied as root dips, used to treat seeds, or inserted into trees or wood products. Biocontrol products currently on the market in the U.S. include Aspire, AQ-10, Galltrol A, Norbac 84C, Bio-Save 10, Bio-Save 11, Blightban A506, Victus, Epic, Kodiak, Deny, Mycostop, Binab-T and W, T-22G and T-22HB, and SoilGard.
Pathogens are controlled by biocontrol agents of the same species or genus as the pathogen in several cases. For example, nonpathogenic Agrobacterium radiobacter is used to control crown gall (Galltrol-A, Nogall, Diegall). Nonpathogenic Fusarium oxysporum is used to control F. oxysporum (Biofox C, Fusaclean) and F. moniliforme (Biofox C). Nonpathogenic Pseudomonas solanacearum controls pathogenic P. solanacearum (PSSOL), while P. fluorescens is used to control P. tolassii (Conquer, Victus). Pythium oligandrum is used to control P. ultimum (Polygandron). These agents may work through antibiosis (A. radiobacter; Kerr, A. “Biological Control of Crown Gall through Production of Agrocin 84,” Plant Dis., 64:25-30 (1980)), competition and induced systemic resistance (Fusaclean; Alabouvette, et al., “Recent Advances in the Biological Control of Fusarium Wilts,” Pestic. Sci., 37:365-373 (1993)), parasitism (Polygandron; Vesely, D. “Germinating Power of Oospores of Pythium oligandrum in a Powder Preparation,” Folia Microbiol., 32:502 (1987)).
Some biocontrol agents control only one pathogen. For example, Conquer and Victus both contain P. fluorescens used to control mushroom blotch caused by P. tolassii. Biocontrol agents are sometimes perceived as serving only niche markets since many products have narrow applicability. In part because of this perception, many biocontrol products are manufactured by small companies. However, most biocontrol agents have multiple pathogen and crop uses. For example, SoilGard controls damping-off incited by Rhizoctonia solani and Pythium spp. on bedding plants and vegetable transplants, as well as Sclerotium rolfsii on carrot and pepper in the field (Lumsden et al., “Biological Control of Damping-off Caused by Pythium ultimum and Rhizoctonia solani with Gliocladium virens in Soilless Mix,” Phytopathology, 79:361-66 (1989); Ristaino et al., “Influence of Isolates of Gliocladium virens and Delivery Systems on Biological Control of Southern Blight on Carrot and Tomato in the Field,” Plant Dis., 78:153-56 (1994); Ristaino et al., “Soil Solarization and Gliocladium virens Reduce the Incidence of Southern Blight in Bell Pepper in the Field,” Phytopathology, 84:1114 (1994)). Some products even control dissimilar pathogens. Deny controls Rhizoctonia, Pythium, Fusarium, as well as several nematodes. BlightBan A506 can be sprayed onto trees, strawberry, tomato, and potato plants to prevent frost damage and fire blight caused by Erwinia amylovora. Trichoderma spp. can control a wide variety of pathogens and appear in more products than any other microbe (Anti-Fungus; Binab T; Supresivit; T-22G and T-22HB; Trichopel, Trichoject, Trichodowels, and Trichoseal; TY). Products containing Trichoderma spp. control species of Amillaria, Botrytis, Chondrostrenum, Colletotrichum, Fulvia, Fusarium, Monilia, Nectria, Phytophthora, Plasmopara, Pseudoperonospora, Pythium, Rhizoctonia, Rhizopus, Sclerotinia, Sclerotium, Verticillium, and wood rot fungi.
Many biocontrol products are applied in agricultural environments of low ecological diversity in order to facilitate establishment of the biocontrol agent. For example, SoilGard and T-22G are mixed with soil-less potting mix. Similarly, Anti-Fungus is applied to soil following steaming or fumigation. Other biocontrol agents are used to protect plant parts. Galltrol-A, Nogall, Diegall, and Norbac 84C are all applied as root dips at transplant to prevent crown gall. Aspire, Bio-Save 10, and Bio-Save 11 are applied post-harvest to citrus or pome fruits to protect these fruits from post-harvest diseases. Several biocontrol agents, including Blue Circle, Epic, Kodiak, and T-22HB, are applied as seed treatments. Binab is applied by spraying, mixing with soilless potting mix, painting on surfaces or inserting pellets into wood to control rot in wood and wood products. Mycostop is applied as a spray, drench, or through irrigation.
In order for biocontrol to be a useful component of an integrated pest management system, research is needed in several critical areas. This integrated approach will rely on accurate assessments of populations of pathogens present in an agricultural field and knowledge of economic thresholds for pathogen damage. Research needs to be aimed at an understanding of ecological parameters important for crop production and survival and efficacy of biocontrol agents, and at identifying and developing new biocontrol agents for control of plant diseases. Knowledge of the biology and ecology of the biocontrol agent, pathogen, and host plant can help to exploit strengths or weaknesses of these organisms to improve control performance. Similarly, knowledge of the ecological, biological, and physical conditions needed for successful biocontrol will permit optimization of these conditions to achieve the best possible levels of control.
The influence of the host plant on the composition and size of microbial communities has received little attention thus far. Larkin and coworkers (Larkin et al., “Ecology of Fusarium oxysporum f. sp. Niveum in Soils Suppressive and Conducive to Fusarium Wilt of Watermelon,” Phytopathology, 83:1105-16 (1993); Larkin et al., “Effect of Successive Watermelon Plantings on Fusarium oxysporum and other Microorganisms in Soils Suppressive and Conducive to Fusarium Wilt of Watermelon,” Pathology, (1993)) reported a cultivar-specific rhizosphere effect on soil and rhizosphere microbial communities associated with different watermelon cultivars. One cultivar in particular, Crimson Sweet, promoted the development of microorganisms antagonistic to the Fusarium wilt pathogen. More research is needed to determine the role of this type of interaction in the enhancement of biocontrol.
One barrier to acquiring an understanding of soil microbial systems has been the lack of suitable techniques for assaying soil samples. Population sizes of many soil microbes, especially fungi, are difficult to measure accurately for several reasons. The term “colony forming unit” reflects the fact that colonies arising on a plate may have come from, for example, microconidia, macroconidia, chlamydospores, ascospores, hyphal fragments, or other propagules. Further, the efficiency of recovery of propagules may differ from one soil to the next. In some cases, such as with Fusarium spp., the pathogens cannot be distinguished morphologically from the nonpathogens. In addition, many microbes are not easily cultured on standard media, although they may play significant roles in disease suppression, as with the mycoparsite Sporidesmium sclerotivorum for control of lettuce drop (Adams et al., “Economical Biological Control of Sclerotinia Lettuce Drop by Sporidesmium sclerotivorum,” Phytopathology, 80:1120-24 (1990)). Finally, even when propagule numbers can be accurately estimated, the effectiveness of these propagules is dependent on their nutritional status and on the types and population sizes of other microbes present in the soil system. All of these shortcomings are compounded by the difficulty of sampling, particularly sampling of microsites. Research is needed to develop rapid, reliable, precise techniques for assaying soil microbial communities.
In the future, research should emphasize combinations of two or more biocontrol agents, since combinations may provide more consistent or more efficient control than a single biocontrol agent. For example, biocontrol agents with different optimal environmental conditions, or biocontrol agents with different mechanisms of action could be combined. Biocontrol agents may even act synergistically such as the combination of Fusarium oxysporum with Pseudomonas spp. to control Fusarium wilt (Lemanceau et al., “Biological Control of Fusarium Diseases by Fluorescent Pseudomonas and Non-pathogenic Fusarium,” Crop Prot., 10:279-86 (1991)). Research is also needed on combining biocontrol agents with other control methods. For example, sublethal heat (solarization) or pesticide stress may weaken a pathogen, making it more vulnerable to the action of biocontrol agents (Lifshitz et al., “The Effect of Sublethal Heating on Sclerotia of Sclerotium rolfsii,” Can. J. Microbiol., 29:1607-10 (1983); Tjamos, et al., “Detrimental Effects of Sublethal Heating and Talaromyces flavus on Microsclerotia of Verticillium dahlias,” Phytopathology, 85:388-92 (1995)). Suitable systems also need to be developed for production, formulation and delivery of biocontrol agents, because these processes can greatly affect efficacy of the biocontrol agent.
Despite the existence and use of biocontrol agents in agriculture, there continues to be a need for development of new plant biocontrol agents. The present invention is directed to fulfilling this need.